This invention relates to a novel flame-retarded thermosetting resin composition for use in the manufacture of various electric and electronic parts such as electrical laminates and prepregs that require excellent dielectric and flame-retardant characteristics. The resin composition of this invention is also useful for bonding, coating, casting, potting and molding purposes where such characteristics are required in the thermoset resin used in electric and electronic parts. The term "electrical laminates" as used herein refers to unclad insulation boards for mounting various parts thereon, as well as metal clad laminates for use in the manufacture of printed circuit boards.
Recently, frequencies used in electronic communication equipment, computers and the like are reaching a high frequency region such as the megahertz (MHz) or gigahertz (GHz) regions. Insulation materials used in such high frequency regions are required to have a low dielectric loss tangent as well as a low dielectric constant. To this end, a variety of resins having a low dielectric constant and low dielectric loss tangent have been developed. Among them, cyanate ester resins are excellent in these dielectric properties. Bisphenol A dicyanate ester resins, for example, have been used in fabricating electrical laminates, while Japanese Laid Open Patent Application No. 250359/1988 discloses a fluorine-containing dicyanate ester for producing thermosetting resins having excellent dielectric properties.
Electrical laminates often require a flame retardancy of V-O in the UL94 standard. The prior art dicyanate ester resins, including just mentioned resins, have excellent dielectric properties compared to epoxy, polyester, phenol and polyimide resins which are conventionally used in the manufacture of electrical laminates. However, their dielectric loss tangent is not fully satisfactory in high frequency regions particularly in the GHz region. Cyanate ester resins with a flame retardant additive having a flame retardancy of V-O in the UL94 standard generally have a dielectric loss tangent greater than that of the corresponding non-flame retarded dicyanate ester resins and, therefore, they are not suitable as matrix resins of electrical laminates used in the GHz telecommunication field. The fluorine-containing dicyanate esters disclosed in Japanese Laid Open Patent Application No. 250359/1988 may provide resins having a flame retardancy of V-O but they are too expensive for practical applications. It is desirable for electrical laminates used in the telecommunication field to have a dielectric loss tangent less than 0.005 and preferably less than 0.004. It is also desirable for the matrix resin of such laminates to have a dielectric loss tangent less than 0.006, and preferably less than 0.005 in the GHz region. For computer applications, the electrical laminates should desirably have a dielectric constant less than 4.0 in the GHz region to enable faster operation. These laminates are often composed of glass cloth substrates of E glass. In this case, the matrix resin should desirably have a dielectric constant less than 3.0, and more preferably less than 2.9.
Moisture absortion is an important factor which adversely affects the dielectric properties in the high frequency region, and in the GHz region in particular. The prior art dicyanate ester resins tend to have relatively high moisture adsorption.
Also required for the matrix resin of electrical laminates are good heat stability and a high glass transition temperature (Tg). These characteristics are important for the dimensional stability and reliability of printed circuit boards (PCB's) made therefrom. For use as multilayer PCB's, the matrix resin should have a Tg of higher than 160.degree. C., preferably higher than 170.degree. C., and most preferably higher than 180.degree. C., measured by the TMA method. Resins having a low dielectric loss tangent in the GHz region and a high Tg are not yet known.
It is, therefore, an object of this invention to provide a thermosetting cyanate ester resin composition having a low dielectric constant and a low dielectric loss tangent in the GHz region in particular, as well as a satisfactory flame retardancy, a low moisture absorption and a high Tg. It is another object of the present invention to provide an electrical laminate and prepregs therefor having excellent dielectric characteristics in the GHz region as well as a low moisture absorption and a satisfactory flame retardancy. Other objects and advantages of the present invention will become apparent as the description proceeds.